This invention relates to an analogue-to-digital converter, such as a sigma-delta modulator.
An example of a first order sigma-delta modulator is shown in FIG. 1. The sigma-delta modulator is an analogue-to-digital converter (ADC). It is configured to convert an analogue input signal 101 into a digital output signal 107. The sigma-delta modulator further comprises a summation unit 102, an integrator 104 and a quantiser 105. The conversion uses error feedback. The output signal is converted into an analogue signal 103 by digital-to-analogue (DAC) converter 106, and then subtracted from input signal 101 by summation unit 102. The sigma-delta modulator is configured to sample at a higher frequency than the input signal. The output takes the form of a high frequency stream of ones and zeros, with the relative proportion of ones to zeros representing the amplitude of the input signal.
A sigma-delta ADC is a feedback system. An important issue with feedback systems is achieving stability. The over-riding requirement is that the phase shift is less than 180 degrees when the loop gain goes through 1 (0 dB). It is usual to design for a 90 degree phase shift, conveniently achieved by a “20 dB/decade” slope for the gain/frequency plot at a gain of 0 dB. An example of such a plot is shown in FIG. 2. The purpose of the feedback system is to suppress errors generated inside the loop via the loop gain. For a sigma delta ADC, the major error is due to the single bit quantisation. This “quantisation noise” (or Q noise) is suppressed at any given frequency by the loop gain. For example, in FIG. 2, the Q noise at frequency 0.1 is suppressed by 80 dB. Typically, in this low frequency part of its spectrum, an oversampling ADC is a useful device that gives a good signal to noise ratio. The noise left in at higher frequencies does not contain the wanted signal and can be digitally filtered out.
An example of a practical scenario in which sigma-delta modulators, are frequently deployed is in RF receivers. RF signals are typically mixed down to a lower frequency after receipt to make them easier to handle. In many receivers the mixed-down signal is fed into an ADC and then digitally processed. One simple, low power way of mixing is to use an analogue switch and switch the RF signal at some other frequency with gains of +1 and −1.
Either the +1 or the −1 signal might be used by the receiver but, in practice, mixing is a lossy process (it degrades signal to noise ratio) so it is preferable to use both the +1 and −1 versions. This might be achieved by feeding both versions into an ADC with a differential input. In this scenario the +1 version is subjected to +n gain and the −1 gain version is subject to −n gain. Many receivers, however, include circuitry that is inherently single-ended and thus not equipped to handle differential inputs. There is therefore a need for an improved ADC for handling both the +1 and −1 versions of a mixed signal.